Confirm the exact IP or domain and the exact list reporting it.
Read that operator’s listing reason and criteria.
Investigate for compromised accounts, open relay, application abuse, malware, bad list acquisition or other outbound anomalies.
Contain and remediate the cause before requesting removal.
Do not pay random third parties promising universal delisting. There is no single global email blocklist.
After removal, keep monitoring the affected sender because relisting usually means the root cause remains.